Leave flags in place if not true|false|undefined.

With this change we will actually have a fully detailed `features.json`
in Ember's master branch. This allows us to have features enabled that
are enabled in the beta branch, but still leave features in the build
for others to enable via ENV.
Merge pull request #3 from rjackson/allow_features_to_exist_if_not_false

Leave flags in place if not true|false|undefined.